First of all, make sure that your login and password are correct.
If the text field shows the "User not found" error, check your other email addresses and aliases, in case you used a different one. You can also look for the GOG account creation confirmation email in your mailboxes, and check what address it was sent to, exactly. If none of the email addresses work, contact our Support team.
If you seem to be using the correct login credentials, but still cannot log in, try these troubleshooting steps:
wipe your browser's cache and cookies, and reload the GOG.com page,
try a different browser altogether (ideally a freshly-installed browser, with no addons/plugins),
add GOG.com to your firewall/antivirus software's whitelist,
contact us if all else fails.